Package on Package (PoP) is an integrated circuit packaging technique to allow vertically combining discrete logic and memory ball grid array (BGA) packages. Two or more packages are installed on top of one another, i.e. stacked, with a standard interface to route signals between them. This allows higher density and smaller physical size for electronic systems such a mobile telephone or personal digital assistant (PDA). There are two widely used configurations for PoP: pure memory stacking where two or more memory only packages are stacked on top of each other; logic package in the bottom with a memory package on top. For example, the bottom could be an application processor for a cell phone. The logic package is typically placed on the bottom because it requires many more BGA connections to the motherboard.
Various standards are being promulgated for use in PoP packaging applications. For example, JEDEC Standard JESD2009-2 defines a Low Power Double Data Rate 2 (LPDDR2) specification for a ball grid memory package. The standard defines features, functionalities, AC and DC characteristics, packages, and ball/signal assignments. This specification covers the following technologies: LPDDR2-S2A, LPDDR2-S2B, LPDDR2-S4A, LPDDR2-S4B, LPDDR2-N-A, and LPDDR2-N-B. The purpose of this specification is to define the minimum set of requirements for JEDEC compliant 64 Mb through 8 Gb for ×8, ×16, and ×32 synchronous dynamic random access memory (SDRAM) devices as well as 64 Mb through 32 Gb for ×8, ×16, and ×32 for non-volatile memory (NVM) devices.
This standard includes a “data not valid” (DNV) signal that may be used by some memories to report a read error in the case the memory includes error code detection (ECD), or to ask controller to re-send the same read command (retry mode) because some memories after having detected an error with error code detection, can correct the data using error code correction (ECC), but it takes some additional time in the form of extra clock cycles. The DNV signal may be asserted for either read or write transactions, as needed.